1. Field of the Invention
The present invention relates to sensors of pressure, acceleration, liquid level, and the like for use in the control of automotive engines. More particularly, it concerns such sensors made of ceramic materials.
2. Discussion of the Related Art
SAE Report 860474 discloses a sensor in which a ceramic diaphragm is bonded to a ceramic support using glass. However, this type of sensor suffers from several disadvantages including inadequate strength of the joints between the diaphragm, the glass, and the ceramic support due to a thermal expansion mismatch between the glass and the ceramic materials, and deformation due to the expansion mismatch in the absence of pressure application at elevated temperatures.
In an attempt to solve these problems, Japanese Patent Unexamined Publication Nos. Sho. 57-4531 and Sho. 63-292032, propose a process for fabricating a durable sensor in which a diaphragm and a ceramic support are first assembled in a raw state, in which the ceramic parts are in the form of unsintered green sheets, and then the assembly is sintered. According to this method, the aforementioned problems can be solved to some extent by avoiding the use of glass. However, there is a need to use substantially the same material for both the diaphragm and the ceramic support and to permit them to shrink by the same amount during sintering. To meet these requirements, very cumbersome procedures of quality control have been necessary in the manufacturing process.
When two green sheets are superimposed on each other and sintered, warpage or bends will result in the sintered assembly unless the two sheets shrink equally during sintering. If a warped sintered assembly is to be used as a pressure sensor for emissions from automobiles, leakage of the exhaust gas can occur, or if vibrating emissions are introduced, a resonance results and makes it difficult, if not impossible, to attain accurate pressure detection.
In order to avoid these problems, the diaphragm and the support used in the pressure sensor must be made of materials that will experience the smallest possible amount of warpage. However, even if the green sheets assembled as a laminate are made of the same material, the amount of shrinkage during sintering will differ from one lot of manufacture to another on account of process variations in sheet fabrication and temperature differences that occur upon calcination, for example. These factors may cause an unavoidable difference in shrinkage of about .+-.0.4%.
If a thick-film process, such as printing, is used, not only does a large difference in shrinkage occur between a sheet and a printed pattern during sintering, but also, the occurrence of warpage remains due to formation of metallic parts for electrodes and leads by printing.